1. Field of the Invention
This invention relates to the compensation of frequency offsets between a modulated carrier signal and a local oscillator signal, and more particularly to frequency offset compensation which is effected by adjusting the demodulated signal phase.
2. Description of the Related Art
The present invention has particular application to the Bluetooth™ wireless technology data communication system, although it is not limited to this technology. The Bluetooth specification, established by the Bluetooth SIG, Inc., integrates well-tested technology with the power-efficiency and low-cost of a compliant radio system to enable links between mobile computers, mobile phones, portable hand-held devices and the like, and connectivity to the Internet.
Part of the Bluetooth physical layer specification calls for binary frequency shift keying (FSK) with a modulation index of 0.28–0.35 to be used as the modulation method, the term “modulation index” being defined as the peak-to-peak frequency deviation in the modulation signal divided by the modulation's data rate. The modulation data rate for the Bluetooth wireless technology is 1 Mb/sec, which yields an allowable peak-to-peak frequency deviation of 280–350 kHz. The Bluetooth specification also allows for the transmitter's frequency to be accurate to within +/−75 kHz, thus permitting a maximum possible frequency offset (FO) between the transmitted carrier frequency and the receiver's local oscillator (LO) frequency as high as 150 kHz. This is larger than the minimum peak frequency deviation of 140 kHz (half the peak-to-peak frequency deviation). Without compensation for this error, the receiver can detect the incoming data 1 and 0 bits as all ones, or all zeros. This would effectively generate a 50% bit error rate (BER), as opposed to the 0.1% BER Bluetooth sensitivity specification.
A prior frequency offset compensation technique in the form of DC offset compensation for FM discriminators is described in National Semiconductor Application Note No. 908, “Specification For The DECT Ari 1™ Interface To The Radio Frequency Front End”, September 1993. This circuit seeks to recover the mean DC level of the input demodulated data signal, based upon a filtered version of the input data stream. The accuracy of the DC recovery depends upon the DC content of the incoming data (which is zero in the case of the preamble for a non-offset Bluetooth technology data packet), and also upon the time constant of an RC filter used in the circuit. The longer the time constant, the more data bits must be received before (the correct DC value is achieved. Once the DC value has been acquired it is held on a capacitor, removing any further dependence upon the DC content of the incoming data. This circuit has proven effective in a system in which the incoming preamble had 32 bits with zero DC content. With the Bluetooth wireless technology, however, the preamble is only 4 or 5 bits long, which can lead to errors with this approach. With such a short time constant, the circuit would significantly degrade the receiver's performance by having too great a dependence upon the incoming data's most recent value.
An improved version of the DC offset compensation scheme is disclosed in “Switched DC Offset Compensation And LO Tuning For Frequency Offset For FM Discriminators For Bluetooth”, presentation by Parthus, 2 May 2001. In this technique, as with the Application Note just described, a DC voltage level varies in accordance with the amount of FO. However, the DC loop has a switched bandwidth which automatically changes based upon the difference between a new average DC value and the existing DC value. The loop also provides for compensating the FO by tuning the receiver's LO to narrow the FO between the received signal and the LO. While this reduces the amount of DC offset at the demodulator output, the system is best used with a frequency discriminator in which the FO directly translates to DC offset.